The fish species Cyprinidon artifrons, Floridichthys carpio, and Gambusia yucatana inhabit shallow mangrove ponds off the coast of Belize. Portions of these ponds experience a diurnal temperature change from 26 C at night to 40 C and above during midday. Repeated field observations indicate Cyprinidon prefer the warmer (and much larger) portions of the ponds whereas the other two species stay in the cooler areas. The hypothesis that temperature is serving as a cue for partitioning within the ponds was supported by laboratory thermal gradient tests in which Cyprinidon preferred temperatures clearly higher than the other two species. The critical thermal maximum (CTM) was determined for the three species using members that had been acclimated to either a daily cycling temperature similar to that for the ponds, or to the mean of the 24-hour cycle (30 C). Cyprinidon acclimated to the cycling temperature had a CTM of 45.5 C, which apparently sets a new record for fish CTM. Acclimation to a constant 30 C lowered the CTM to 43.7 C. Floridichthys and Gambusia acclimated to the cycled temperature had CTMs of 43.9 and 43.3 C respectively, and 42.5 and 42.6 C for those acclimated to 30 C. All three species appear to have the ability to tolerate the high temperatures throughout the ponds but only Cyprinidon utilize the whole pond during the day. (Copyright (c) 1993 Kluwer Academic Publishers.)